Method and apparatus for enhancing vertical chrominance resolution

ABSTRACT

Chrominance component signals of two different types are produced such that the chrominance signals alternate within a field and between adjacent fields in order to provide an improved amount of information from which to enhance the vertical chrominance resolution of a television receiver. The appropriate chrominance structure is either provided by a special source or else a conventional source is filtered in order to produce the appropriate structure. The filter comprises two portions, one for filtering chrominance signals of a first type from a first field and one for filtering chrominance signals of a second type from the first field in order to provide interpolated signals. A specific filter structure is disclosed which may be used for either pre- or post-filtering.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the transmission and reception of colortelevision signals in component form and more particularly to a methodan apparatus for handling the chrominance components of the signals.

The present invention will be described in relation to a componenttelevision signal known as a Multiplexed Analogue Component (M.A.C.)signal although this is but one application of the present invention.

SUMMARY OF THE INVENTION

The present invention provides a method of assembling chrominancecomponents signals comprising providing a plurality of fields of videosignals in which the two different types of chrominance componentsignals alternate within each field, and arranging the fields so as toproduce a resultant chrominance structure which is noninterlaced andcontains alternate chrominance component signals both within each fieldand between adjacent fields, whereby to permit enhanced verticalchrominance resolution.

Features and advantages of the present invention will be apparent fromthe following description of an embodiment thereof when taken inconjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows diagrammatically the line structure for the chrominancecomponents of a MAC-type signal;

FIG. 2 shows a graph representing the repeat spectra in vertical andtemporal frequency produced by the line structure of FIG. 1;

FIG. 3 shows diagrammatically the proposed type of line structure;

FIG. 4 shows a graph representing the repeat spectra in vertical andtemporal frequency produced by the line structure of FIG. 3;

FIG. 5 shows a diagrammatic representation of a filter producing theline structure shown in FIG. 3; and

FIG. 6 shows the characteristic of the filter shown in FIG. 5.

DETAILED DESCRIPTION

MAC is defined to code the two color difference components (U and V inEurope) on alternate lines with a frame-reset sequence. This sequence isillustrated in FIG. 1.

There are sound reasons why this sequence has been selected in place ofthe alternative four-field sequence which is not reset each frame. Themain reason is the residual alias components in the frame-reset case aremuch less disturbing. In addition, if such a signal were ever to beprocessed in the studio then its two-field sequence would be easier tohandle than a four-field sequence. An implication of using this sequencehowever is that the vertical chrominance resolution is limited to aquarter of the luminance vertical resolution capability. This is becauseeffectively there are only 144 (575/4) vertical samples of each colourin an active picture scan whereas there are 575 active vertical samplesof luminance. In order to achieve the full vertical resolution affordedby the 575 lines, field stores are required in the receiver.Nevertheless even if this complexity is allowed for color the verticalchrominance resolution is still limited to that offered by 144 lines.

In frequency terms this limitation is evident by considering the repeatspectra (in vertical and temporal frequency) generated by this alternateline sequence, as illustrated in FIG. 2.

The open circles are those repeats arising from the 625 interlace scan.The extra repeats introduced by alternate line omission in a frame resetfashion are illustrated by the shaded circles. These repeat spectra arehalf-amplitude but nevertheless they impose a Nyquist-limiting verticalchrominance frequency of 72 cycles per picture height (equivalent to 1.8MHz horizontally). In practice the chrominance vertical resolutionobtained after pre- and post- filtering is equivalent to 1.1 MHzhorizontally (-3 dB).

If we wish to maintain a frame-reset sequence for transmission,techniques for achieving a vertical chrominance resolution greater thanthe Nyquist limit described above are not obvious. Nevertheless thefollowing line of thought gives a clue to a possibility: with thecurrent approach there appears to be a paradox. Although there are halfas many lines of each colour difference component as there are ofluminance, the available vertical resolution is only a quarter as great.As we have seen, this is due to the sampling structure formed by thechrominance lines. If we conceive of a chrominance line structure whichis analogous to the luminance structure, except that it is halved indensity vertically, then we arrive at a 3121/2-line interlace structure.Two such structures (one for each colour difference component) areillustrated interleaved in FIG. 3.

In this case there are 288 (575/2) vertical samples of each colourdifference component, just as there are 575 for the luminance. Infrequency terms, the repeat spectra appropriate to such a line-structureare shown in FIG. 4.

In this case the "Nyquist-limit" vertically is 144c/ph (equivalent to3.7 MHz horizontally). As with the luminance, field store processing isrequired in order to achieve this increased resolution.

The relationship between the line-structure of FIG. 3 and the MACframe-reset structure of FIG. 1 is a simple one. A vertical shiftupwards of alternate fields in the interlace structure (FIG. 3) by a"frame-line" (i.e. 1/575 of a picture height) yields the frame-resetstructure (FIG. 1). It is thus possible for a 3121/2 line interlacedcolour structure (FIG. 3) to be transmitted in a 625-interlacealternate-line frame-reset format (FIG. 1). A "conventional receiver"would process these colour lines as a true frame-reset structure (thusalternate fields would have a minor vertical shift) while a higherdefinition receiver would treat the lines as being part of the 3121/2line interlaced structure and thus achieve an increased verticalresolution.

A higher definition receiver would receive the FIG. 3 chrominancestructure and undertake scan conversion to derive a 3121/2 linenon-linterlaced structure. This provides a greater basis of informationfor subsequent vertical interpolation to provide a 625 linenon-interlaced chrominance structure for combination with the 625 linenon-interlaced luminance structure such a receiver would also produce.

If the source of chrominance signals is a special source such as aspecial camera, it is possible for the chrominance structure as shown inFIG. 3 to be available directly from the source. If the source scanswith the basic 625 line inter-laced structure then the lines inalternate fields of the colour structure of FIG. 3 are not available. Inthis case, therefore, values need to be interpolated. This interpolatorcan be the prefilter which shapes the vertical-temporal frequencyspectrum of the color components. As an example such a prefilter hasbeen designed which would offer a -3 dB vertical bandwidth equivalent to2.14 MHz (on stationary or horizontally moving pictures). The filtercoefficients are defined on a grid of 625-sequential-scan lines asillustrated in FIG. 5a, although in practice it will mean that "reallines" are filtered by the coefficients of FIG. 5b and "interpolatedlines" are provided by using the coefficients of FIG. 5c. Thevertical-temporal frequency response of such a filter is illustrated inFIG. 6. If a similar post-filter is used prior to sequential i.e.non-interlace display in a higher definition receiver then the combinedfilter responses give a vertical resolution for stationary orhorizontally moving scenes equivalent to 1.9 MHz (-3 dB). For verticallymoving pictures this is reduced and in the worst case (having a 25 Hztemporal frequency component) the combined response gives a resolutionequivalent to 0.6MHz (-3 dB). This is of course just an example. Adifferent filter could be used or indeed one or both of the filterscould be adaptive.

The effect on a conventional receiver would be primarily that of lesssevere pre-filtering. The results obtained would be more like thoseobtained with a 1-1 pre-filter rather than with a typicalalias-rejecting seven-tap pre-filter.

By using the presently proposed invention, a 625 line MAC system canprovide a vertical chrominance resolution that approaches, if not equalsthat of a 1125 line H DTV system in its transmission format.

I claim:
 1. A method of producing a color television signal in componentform, comprising the steps of:scanning a scene; generating luminance andtwo types of color difference component signals representing the scenesuch that each luminance or color difference component signal relates toa line in one of a plurality of fields of a television display of thescene, the luminance component signals relate to every other line in onefield of each frame and to every other line in the other field of eachframe of the television display of the scene, said every other line inone field being interlaced with said every other line in the otherfield, the color difference component signals relate to every other linein each field of the television display of the scene, the colordifference component signals relating to each every other line in eachfield alternate in type, the color difference component signals relatingto lines in said one field of each frame relate to the same lines as theluminance component signals relating to said one field of each frame ofthe television display of the scene, the color difference componentsignal relating to the first line in said one field of each frame is ofa different type from the color difference component signal relating tothe first line of said other field of each frame of the televisiondisplay, and the color difference component signals relating to thefirst of said every other lines in each frame are controlled to be ofthe same type: time multiplexing luminance and color differencecomponent signals to form a color television signal in component form;and the generated color difference component signals relating to saidother field of each frame of the television display relate to differentlines from the luminance components relating to said other field of eachframe.
 2. A method according to claim 1, wherein the scanning stepcomprises raster scanning the scene using a two-field line-interlacedscan, and the generating step comprises generating luminance and twotypes of color difference component signals indicative of parameters ofthe scanned lines, and passing the color difference component signalsthrough a filter to alter the lines in said other field of each frame ofa television display of the scene to which the color differencecomponent signals relate.
 3. A method according to claim 2, wherein thestep of passing the color difference component signals through a filtercomprises passing the color difference component signals through afilter acting on the vertical temporal frequency spectrum of the colordifference component signals.
 4. A method of receiving a colortelevision signal in component form comprising time-multiplexedluminance and color difference component signals representing a scene,each luminance or color difference component signal relating to a linein one of a plurality of fields of a television display of the scene,comprising the steps of:receiving a color television signal in componentform; demultiplexing the luminance and two types of color differencecomponent signals in the received signals, the demultiplexed luminancecomponent signals relating to every other line in one field of eachframe and to every other line in the other field of each frame of thetelevision display of the scene, said every other line in one fieldbeing interlaced with said every other line in the other field, thedemultiplexed color difference component signals relating to every otherline in each field of the television display of the scene, thedemultiplexed color difference component signals alternating in typewithin each field, the demultiplexed color difference component signalsrelating to the same lines in said one field of each frame of thetelevision display as the demultiplexed luminance component signals, thedemultiplexed color difference component signal relating to the firstline in said one field of each frame being of a different type from thedemultiplexed color difference component signal relating to the firstline in said other field of each frame of the television display, andthe demultiplexed color difference component signals relating to thefirst of said every other line in each frame being controlled to be ofthe same type; outputting the luminance and two types of colordifference component signals to a television display; the demultiplexedcolor difference component signals relating to said other field of eachframe relate to the same lines as the color difference component signalsrelating to said one field of each frame of the television display ofthe scene; and further comprising the step of interpolating using thedemultiplexed color difference component signals to obtain further colordifference component signals, the further color difference componentsignals relating to lines in the frame different from the demultiplexedcolor difference component signals.